There has been recently increasing an in-vehicle internal combustion engine using a variable valve apparatus which changes valve opening/closing characteristics such as valve timing or a valve lift amount of an intake valve or an exhaust valve for the purpose of a power output improvement, a fuel economy improvement, reduction of exhaust emissions and the like. There is, however, a problem with an occurrence of variations in responsiveness of variable valve control caused by variations in the manufacture of the variable valve apparatus or variations in performance of the variable valve apparatus due to aging.
Japanese Unexamined Patent Publication No. 9-256878 has, for solving this problem, disclosed a variable valve timing apparatus which calculates a changing speed of valve timing at the time of maintaining a control input (control duty of a hydraulic control valve) of the variable valve timing apparatus at a predetermined value and corrects the control input of the variable valve timing apparatus so as to reduce a difference between the changing speed of the valve timing and a predetermined speed.
In addition, Japanese Unexamined Patent Publication No. 2002-155766(U.S. Pat. No. 6,386,158B1) has disclosed a variable valve timing apparatus which sets a reference model which simulates characteristics as a reference of the variable valve timing apparatus and calculates a control input of the variable valve timing apparatus so that a difference between an output of the reference model and an actual valve timing of the variable valve timing apparatus is reduced.
In general, as shown in FIG. 4, in a variable valve timing apparatus of a hydraulically driven type, a changing speed (advance speed or retard speed) of valve timing changes with a control input (control duty of a hydraulic control valve). It is found out, however, according to the recent experimental result of the inventors, that the variable valve timing apparatus is characterized in that in a region where the changing speed of the valve timing becomes substantially a maximum, variations in the changing speed of the valve timing get small and in a region other than the above region, variations in the changing speed of the valve timing get relatively large.
However, since, in the variable valve controls as described above, the characteristic as a reference thereof is uniformly without consideration of the feature in regard to variations in performance of the variable valve timing apparatus, the characteristic as the reference is required to be set for adaptation to a variable valve apparatus as a lower limit product on variations in performance for realization of the characteristic as the reference even in the variable valve apparatus which is the lower limit product on the variations in the performance. As a result, there occurs the problem that responsiveness of the variable valve control is sacrificed for compensation for the variations in performance of the variable valve apparatus.